1. Field of the Invention
The present invention relates to an excitation light source to be used for Raman amplification and a Raman amplifier and an optical transmission system using the excitation light source.
2. Description of the Related Art
In recent years, a positive progress has been made in the development of ultrahigh-speed WDM (wavelength division multiplexing) optical transmission systems of 100 Gbps and above using a digital coherent receiving method. The digital coherent receiving method provides the following distinctive advantages:
Receiving sensitivity can be improved by the detection of signal light through mixing with local light.
The wavelength dispersion and polarization mode dispersion can be compensated for by use of a digital circuit.
With these advantages, the systems using the digital coherent receiving method can realize long-distance transmission of ultrafast signals which have previously been impracticable. Moreover, such systems no longer require wavelength dispersion compensation using dispersion compensation fibers and thus allow transmission even through optical transmission path of inferior quality with much polarization mode dispersion (PMD).
In a polarization mode dispersion, differential group delay (DGD) occurs between two polarization modes orthogonal to each other of light transmitting through an optical fiber. The DGD is caused by random birefringence that occurs due to slight distortions of the core of optical fiber or stresses from the outside (e.g., environmental temperature change or mechanical vibration). The DGD, which indicates the amount of separation of two signal components having separated within the optical fiber, can vary greatly with wavelength or time. The polarization mode dispersion is an average value of DGD with respect to wavelength and time.
In a WDM optical transmission system, a high optical signal-to-noise ratio (OSNR) must be achieved if a high-speed optical transmission at 100 Gbps is to be accomplished with a quality equivalent to that of the optical transmission at 10 Gbps or 40 Gbps. In the WDM optical transmission system, an erbium-doped fiber amplifier (EDFA) may be used to amplify optical signals that have been attenuated. Yet, if the transmission distance is long, the input level of the EDFA can drop, which causes a drop in the optical signal-to-noise ratio. So when the transmission distance is long, a backward excitation Raman amplifier is introduced. A Raman amplifier is a distributed amplifier that acts on optical fiber transmission path itself as the amplification medium. When a Raman amplifier is used, the lowest power of signal light in the optical transmission path can be maintained higher than when the Raman amplifier is not used. This will limit the drop in the optical signal-to-noise ratio (see Reference (1) in the following Related Art List, for instance).